Cracking of petroleum vapors



NOV. 13, 1934. A MCG, WQQD 1,980,462 CRACKIG 0F ETRoLEUM vAPoRs v FiledJan. 14. l1931 k, @Y @E V 'ne Fur/7 ace ANO/@fw Ms? 69560@ WOOD INVIA/meBY /775 Arme/Vey.-

Patented Nov. 113,

UNITED STATES 1,980,462 CRACKING oF PETROLEUM vARs Andrew McGregor Wood,Miri, Sarawak, Borneo Application January 14, 1931, serial No.5138,'17'1 8 Claims.

This invention relates to the conversion of heavier hydrocarbons intolighter hydrocarbons by heat and particularly'to such cracking carriedout in the vapor phase.

Cracking of "petroleum in the vapor Yphase has heretofore beenhandicapped by difficulties -oaused by the method by which the vapor` isbrought to and maintained at the cracking temperature. "Due tothe verydispersed molecular 10 nature of a gas andthe consequent lowconductivity, it is almost impossible under existing methods to raise toandl maintain a body of vapor at cracking temperature in such way thatsaid temperature willbe uniform throughout the body,

A and to avoid a considerable temperature'gradient between lthat 'partof the vapor `next to the heat supply and'that part more remote lfreinit. For instance, if the vapor is heated in furnace tubes, the vaporpassing near theinner circumference of the tube is considerably hotterthan that near the center. Thus to produce a cracking temperature at thecenter the vapor vnear'the tubo wall must needs be overcracked With theconsequent and very undesirable deposition of carbon and reducedeiiiciency and the burning out of tubes. Another diiiiculty is after thevapor has once reached cracking temperature to prevent overheatingduring its further progress through the tubes, as only a comparativelysmall amount of heat is needed to maintain Athis temperature, and it isdiicult to control the input of heat throughout the length of the tubecoil.

My invention `consistsof a method to maintain a. uniform temperaturethroughout a cross section of the vapor at any point in the crackingtube, and alsol a small temperature gradient'along the iiow of vaporsbetween any two points in said tube avoiding sudden raises oftemperature or regions of too high temperature. To obtain this, I pass40 countercurrent to the flow of vapors, through the center of theheating tube a stream of relatively cool liquid oil or vapors through aninner concentric tube; the annular space between the two tubes is thecracking space.

The attached drawing shows by way of illustration or example one formVof apparatus, adapt ed to carry my invention intoeffect: it will beunderstood that I do not limit myself to this par,-y

ticular design. f 1

The more or less diagrammatically shown apparatus consists of t furnace1, evaporator 2, dephlegmator 3, surge tank 4 andy condenser 5. In theoperationof this apparatus; usingV liquid oil in the inner of the twoVconcentric tubes; the

pump 6 forces the Aoil through thepipe '7 into the dephlegmator 3 fromwhich it goes, 'after ther removal of any light fractionsthrough thepipe 8 into the surge tank 4. It is then forced through' the pipe 9by'theipump 10 into acoil l`1`1fin the' furnace l, valve 15 in pipelfbeigfclos'ed. Cil N ll enters concentrically the larger coil 1`2,valve 19 and 20 both being closed. Theliquid oil oon-` tinues in theinner tube ofcoil 1'2-being progressively heated by heat transfer vfromthe outer tube. It now leaves the furnace thrughpipe 13, valve 22beingclo'sed, and enters the evaporator 2 Where, with' the `aid ofsuperheated vsteam from the open 'pipe 31,V vit is. partly. evaporated.The liquidresidue is removed through the'pipe: 14'. The vapor is nowreturned to the furnace through' pipe 16 and'2l and enters 'the gouter.tube :of coil 12. It passes through this tube in turbulent conditionand is here subjected'to crackirig":t'ernr` peratures. Theproducts ofthe reactionarethen passed through pipe v1"? intoA `the rdephlegmator 3Where the heavier'hydr'ocarbons are. condensed andreturned tov the"system, The :gasoline vapors then pass through pipe' 24 into vthecondenser 5 and. thence to storage.

The pipe 18 serves as a means of temperature control for the oilentering coil 12.` If, atany time, the oil becomes too hot, ,it isonlynecessary to open valve 15 slightly, at the same time closing valve25 somewhat, thusdeflecting part'jofthe oil from its path through'thefurnace.. It isrthe.y 8`5 function Aof the cylinder 26 to mix the hotterand cooler oil streams sothat oilI of even temperature enters coil 12.By this arrangement 'a Ivery accurate control isobtained. t

The oil to be treated is heated in coil llrunderl control to atemperature of about. 50W-QF. -:It.;,isv then passed through* the innertube; of coil'12;` Where, due' to heat l.exchange withthe` outer countercurrent W of hot vapors, itis gradually heated to about IOW-750 F.Suiiicientzzpressure;` 9'5 is maintained to keep the` oiljinliq'uidcondition until its release on entering the evaporator. :In some casesit might be desirable to vobtain'some cracking in the liquid phasebefore the oil enters the evaporator; temperaturesiand velocities willthen be `changed accordingly throughoutzithe system and the processoonizluctedVv at ai generally higher degree of heat, so that.the.;1iquid,' 0.115 ,will enter the evaporator at say 900 F. gIf;desired; the gasolineobtained may bev separatediflomgtha heavierfractions before the vapors-fof` thelatter, are passed into the outercracking jcoil.; :I,-I evaporization takesV place gthe'ifprocess.Ybei-ng assisted, if` desiredyby thetllse off-,supelrheatgd steamsupplied by the openfsteam pipeil. Surfin no ioo cient pressure ismaintained on the evaporator to pass the vapor into the outer orcracking tube, where it is cracked, and from there to the dephlegmator 3where the heavy or uncracked fractions are condensed and recycled, thelight fractions going on to the condenser 5 where they are liquefied.'I'he pressure on the condenser s only slight, say lbs., only sufiicientto handle the uncondensed gases. The temperature of the vapors onleaving the evaporator will be around S-650 F.; this will be raised tosay 1000 F. inv

the outer cracking tube.

If it is desired to pass vapors through the inner tube instead of liquidoil, some changes in the operation of the apparatus have to be made.Valve 25 is closed and Valve 19 is opened, thus allowing the hot oil togo directly to the evaporator. After vaporization the volatile matter isreturned to coil 12 through pipe 28 by closing valve 29 and opening 20and the vapors then circulate through the inner tube.L After passingthrough this tube they are returned directly to the outer tube forcracking, this being accomplished by closing valve 30 in pipe 13 andopening valve 22.

When vapor is used in the inner coil instead of liquid oil thetemperatures on the relative parts of the Aapparatus are about the same.'Ihe oil from coil 11 under any necessary pressure to keep it in theliquid state, is forced into the evaporator ,1 at about 600 F. Itreturns as vapor to the inner tube of coil 12 at 500 to 550 F. and onleaving this tube has attained a temperature of about 700- 750 F. In thereturn of the vapor through the outer tube the maximum temperature ofabout 1000 F. is reached.

A uniform temperature throughout the vapor in a cross section of thecracking tube is obtained by effecting a turbulent flow of the vaporsthrough the tube. At a certain criticalvelocity the flow of vaporsthrough a given tube will change from a stream line flow into aturbulent one, which flow will continue for all higher velocities. Thiscritical velocity is variable with the physical characteristics of thetubes. The use of the inner concentric tube restricts thespace in thecracking tube,fthus increasing the turbulency without decreasing theheating surface. The turbulency consists of a whirling motion of thevapors, the centrifugal force of which will tend to throw the cooler,more dense molecules to the outside of the f tube while the hotter,lighter ones will move to- Wards the center. Inv this Way the coolestportions of the vapor will pass nearest the heating surface, Whileoverheating will be prevented by the circulation of the hottest portionsaround the pors start out in thecracking tube they are in contact withthe highest inner tube tempera-tures and as they pass along they aregradually heated by the furnace to, say, about 1000 F. At this point theinner'tube is considerably cooler and j an equilibrium in theheat'transfer is effected so that the inner tube is absorbing as muchfrom the outer tube as the outer tube is from the furnace, thus keepingthe cracking temperature constant. Past this point, the inner tubebeing' still cooler, will be absorbing more heat `from the outer tubethan the outer tube is receiving from the furnace with the consequentgradual decline of the temperature in the outer -tube until at the exitfrom the furnace it is reduced to, say y about 750 F. The inner countercurrent flow in this way permits of a very effective control of crackingtube temperatures due to the ease with which its temperature iscontrolled by valve 15.

By changing the temperature of incoming oils the maximum crackingtemperature can be varied. The point on the tube Where this maximumtemperature occurs can also be thus varied with respect to the ends ofthe tube. This is of value in operation due to the fact that thecorrosional wear is greatest at the point of maximum temperature.A Ifthis point can be shifted the tube will have a longer life before itburns out.

I have specified the above temperatures and method of operation onlyy byway of example. However, I do not limit myself to these, as manytemperature combinations and operating methods are possible, based onthe principleof my. invention and depending on the nature of the oilto-be cracked and the productsV desired.

I claim as my inventionz:

1. A process for the crackingof hydrocarbon oil in the vapor phase'comprising heating a` tubular stream of oil to a temperatureinsufficient for cracking, partlyvaporizingthe oil and separatingresultant vapors from the liquid oil, passing the vapors thus freed fromthe liquid oil again through the heating zone at a cracking temperature,counter current. tothe stream of liquid oil in indirect heatexchanging'relationship therewith, and ,heating` the` vapor .stream tovapor phasecracking temperature during such passage.

2. A process for the cracking of hydrocarbon oil in the vapor phasecomprising heating a tubular stream of oil to cracking temperature,partly vaporizing the oil and separating resultant vapors from theliquid oil,fpassing all or part of the vapors thus freed from the liquidoil again through the heating zone at a cracking temperature, countercurrent to the stream of liquid oil in indirect heat exchangingrelationship therewith, and heating the vapor stream to vapor phasecracking temperature during such passage.

3. A lprocess for the cracking of. hydrocarbon oil in the vapor phasecomprising controlling the cracking temperature of a substantiallyliquidfree tubular stream of hydrocarbon vapor being heated to vaporphase cracking temperatureby passing a countercurrent stream ofrelatively cooler hydrocarbons of a controlled temperature,concentrically through the stream of vapors.

4. An apparatus for cracking hydrocarbon oil in the vapor phasecomprising a furnace, a heatico ics

lio

iis

ing coil therein, an'evaporator to which the heat- .'T ing coil isconnected, an outer'concentric heating coil in the furnace andsurrounding the first heating coil, and means to pass vapors from theevaporator through the outer'coil.

5. In a vapor phase cracking process, the method which comprises passinga stream of hydrocarbon vapors through a heating zone, heating thestream to vaporphase cracking temperature during its passage through theheatingzone by passing a stream of relatively coolerhydrocarbons'throughthe heating zone 'countercurrent to and in indirect heat exchangerelation with the vapor streambeing heated.l

6. In a vapor phase cracking process, the

indirect heat exchange 'with heating gases,and

method which comprises passing an Iannular stream of hydrocarbon vaporsthrough a heating tube, heating the vapor stream to vapor phase crackingtemperature by passing heating gases over the tube, and passing a streamof relatively cooler hydrocarbons centrally through the tubecountercurrent to and in indirect heat exchange relation with the vaporstream.

'7. In a vapor phase cracking process, the method which comprisespassing a stream of hydrocarbon vapors through a heating zone, heatingthe stream to vapor phase cracking temperature during its passagethrough the heating zone by indirect heat exchange with heating gases,and passing a stream of relatively cooler hydrocarbons through theheating zone countercurrent to and in indirect heat exchange relationwith the vapor stream being heated, heating the stream of relativelycooler hydrocarbons to the Vaporizing temperature of a substantialportion thereof,

discharging the thus heated stream into a vapor separating zone andseparating the same therein into vapors and residue, and passing theseparated vapors from the separating zone to the heating Zone toconstitute said stream of hydrocarbon vapors.

8. In a vapor phase cracking process, the method which comprises passingan annular stream of hydrocarbon vapors through a heating tube, heatingthe vapor stream to vapor phase cracking temperature by passing heatinggases over the tube, and passing a stream of relatively coolerhydrocarbons centrally through the tube countercurrent to and inindirect heat exchange relation with the vapor stream, subsequentlyseparating vapors from the stream of relatively cooler hydrocarbons, andpassing the separated vapors to the heating tube to constitute saidannular stream.

ANDREW MCGREGOR WOOD.

